1. ensure per-vertex color on a model is bright (usually RGB 192, 192, 192 is used in game). Select object, then use Surface->paint->Color, expand properties, set color and press "Fill Selected".
2. dif.spec.mult.dif.spec can produce dark look depending on textures you use, try different shader first, may be it will give better result; then try different texture(s), may be original game textures as a test, since the problem might be related to textures too.
Does the problem occur when you switch paint/looks only? have you tried to re-import your model back into ZModeler to see material settings on associated paint/look variants? have you inspected associated .mat files manually? also, have you tried the model in game, may be the game will report more detailed error log related to this case?